1. Field of the Invention
The present invention relates to an actuator for a mobile device, which can enhance a sensitivity characteristic and prevent a tilt of a driving unit caused by a difference in flux density.
2. Description of the Related Art
Recently, camera-integrated mobile terminals such as mobile phones or personal digital assistants (PDAs) are launched on the market. Further, consumers also demand such mobile terminals having a camera with a high pixel resolution and various functions. In such a camera, a lens is attached on an imaging element such as a charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS) so as to image an object, and data of the imaged object is recorded into a predetermined recording medium.
With the recent development of mobile terminals such as portable phones and personal digital assistants (PDAs), the mobile terminals provide a phone call function and are used as multi-convergence devices. The most representative of the multi-convergence devices is a camera module. The resolution of the camera module changes from 300,000 pixels (VGA) to 8,000,000 pixels. Moreover, the camera module provides various additional functions, such as auto-focusing (AF) and optical zoom.
In particular, a demand for mobile terminals having a camera module provided with an auto-focusing function rapidly increases, because the camera module can provide a high-quality image in which focusing is achieved regardless of a focal distance from an object. However, in such a camera module with various functions as well as the auto-focusing function, the number of built-in parts increases. Therefore, the overall size of the camera module inevitably increases, compared with that of a general camera module.
Accordingly, there are difficulties in assembling such a camera module into a mobile terminal.
To overcome such a problem and provide an auto-focusing function to small-sized optical devices, a voice-coil-motor (VCM) type actuator is frequency used, in which a driving unit having a lens module mounted therein is vertically driven by an electromagnetic force generated by an interaction between an electric field generated by a magnet and a magnetic field generated by a coil, thereby performing auto-focusing.
FIGS. 1 and 2 are a perspective view and exploded perspective view of a conventional VCM-type actuator for a mobile device, which has been disclosed in Korea Patent Application No. 2005-47170 filed by the present inventor. Now, the structure and problem of the conventional actuator will be described.
As shown in FIGS. 1 and 2, the conventional actuator 20 for a mobile device includes a holder 21, outer and inner yokes 23 and 25 which are mounted in the holder 21, a coil 27 interposed between the outer and inner yokes 23 and 25, a bobbin 31 coupled to the coil 27 and having a lens module provided therein, a pair of holder PCBs (printed circuit board) 22 which are boned and coupled to both sides of the holder 21, and a pair of suspension wires 35 of which both ends are bonded and electrically connected to the respective holder PCBs 22, the suspension wires 35 being formed in an X shape.
The coil 27 is wound around the outer circumferential surface of the bobbin 31 so as to be disposed inside magnets 29. The bobbin 31 is vertically transferred by an electromagnetic force generated between the coil 27 and the magnets 29.
Each of the holder PCBs 22 coupled to the holder 21 has a predetermined circuit pattern formed thereon, and a current applied from outside through the circuit pattern is applied to the suspension wires 35 coupled to the upper portions of the holder PCBs 22.
The suspension wires 35 formed in an X shape are elastically coupled to both sides of the upper surface of the bobbin 31, while being disposed symmetrically with each other.
The conventional actuator 20 for a mobile device constructed in such a manner can be maintained horizontally, because the suspension wires 35 are disposed symmetrically with each other in both sides of the upper surface of the bobbin 31. Then, only when the bobbin 31 is vertically transferred, a tilt of the bobbin 31 can be prevented.
Further, since the suspension wires 35 are used instead of a typically-used plate spring which is difficult to process, a manufacturing process is simple, and a manufacturing cost can be reduced. However, since the suspension wires 35, which elastically pressurize the bobbin 31 which is vertically driven by the electromagnetic force inside the holder 21, support only the upper portion of the bobbin 31, a posture difference of the actuator 20 may occur. More specifically, when the actuator 20 is vertically maintained, the bobbin 31 droops downward inside the holder 21 due to the weight thereof. Therefore, when the actuator 20 has a posture tilted at a predetermined angle, the bobbin 31 is inevitably tilted.
Accordingly, a distance between the magnet 29 and the coil 27 decreases in a side where the bobbin 31 is tilted, and increases in the opposite side. In this state, when the bobbin 31 is driven by applying a voltage to the actuator 20, the bobbin 31 is further tilted by the posture difference.
In the conventional actuator 20 for a mobile device, it is advantageous that the effective length of the suspension wires 35 is enlarged, in order to enhance a sensitivity characteristic of the suspension wires 35. However, as the effective length of the suspension wires 35 increases, it is difficult to reduce the size of the actuator 20. Therefore, there is a limit in enlarging the effective length of the suspension wires 35.
Further, in order to electrically connect the suspension wires to the holder PCBs 22, a soldering process of the suspension wires 35 is added. In this case, a defect occurrence rate may increase because of flux or foreign matters generated during the soldering process.